A common technique of diagnosing a digital system is by introducing known data patterns into the system and sampling for expected modifications, by the system, of the data patterns at desired nodes. This technique is usually effective in diagnosing a combinational circuit since it involves no memory of preceding states. However, the technique is not as effective in diagnosing a sequential circuit because data at each node is usually dependent upon both the system's current state and its input, so that if a circuit failure causes unexpected state transitions, the sampled result may be so contaminated by having travelled through a wrong system path that analysis of the sampled result most frequently becomes impossible.
In U.S. Pat. No. 4,476,560, which has the same assignee as the present invention, there is disclosed a technique for solving the above identified diagnostic difficulty of a sequential circuit. By providing a circuit that breaks a signal path, with the capability to introduce and remove data signals into and from the signal path, a sequential circuit can then be diagnosed as a plurality of combinatorial circuits.
The block diagram of a prior art part '818 diagnostic register circuit is given in FIG. 1. The circuit has a shadow register interfacing with an external device. The shadow register is insulated from the signal path by an output register. This insulation is provided to prevent possible damages to the system which may occur when the diagnostic data is being input into the shadow register. The insulation also prevents the shadow register from interfering with the normal operation of the system during the introduction of diagnostic data into the shadow register.
In prior art diagnostic register circuits, signal path switching is performed by a multiplexer connected to the input of the output register. The multiplexer performs signal path switching by selectively switching the input of the output register. A major problem associated with the prior art configurations has been the delay of signal propagation caused by the multiplexer logic. Therefore, there is a need for a circuit whereby the multiplexer can be eliminated. It is also desirable to have a circuit whereby operational and functional compatibility with the '818 part device can be preserved.